1. Technical Field
The present invention relates to a switching power supply that converts from an alternating current to a direct current, and in particular to a power factor correction type switching power supply unit that improves a power factor.
2. Related Art
In recent years, a switching power supply unit that has an alternating current voltage as an input has been widely utilized in electronic instruments. This kind of switching power supply unit being one which, by causing a switching operation of a switching element linking an input and an output, converts a full-wave rectified alternating current input voltage into a direct current output voltage of a desired size, and supplies it to a load, for example, the one described in JP-A-2002-176768 (refer to, in that reference, paragraphs [0045] to [0056], FIGS. 7, 8, and the like), to be described hereafter, is known.
FIG. 14 is a circuit diagram showing one example of a heretofore known power factor correction type switching power supply unit. Herein, a power factor correction (PFC) type switching power supply circuit that operates in continuous conduction mode is shown, and this is applied to an active filter type power supply unit.
The heretofore known power factor correction type switching power supply unit shown in FIG. 14 has a full-wave rectifier 4 that full-wave rectifies a commercial power supply 2, and its output is connected to one end of an inductor L1. The connection point of the other end of the inductor L1 and a diode D1 is connected to the drain terminal of, for example, an N-channel type MOS transistor (a metal oxide semiconductor field-effect transistor) configuring a switching element 6. The other end of the inductor L1 is connected to a load 8 via a rectifying and smoothing circuit formed of the diode D1 and a capacitor C1, and a direct current output voltage Vout is output to the load 8.
As well as the source terminal of the MOS transistor, which is the switching element 6, being connected to the ground (GND), the gate terminal is connected to an output terminal DO of a power factor correction control circuit 10A. One end of a series resistor circuit formed of resistors R1 and R2 is connected to the connection point of the full-wave rectifier 4 and inductor L1, and the other end is grounded. A multiplier input terminal VDET of the power factor correction control circuit 10A is a terminal into which a detected value of an output voltage of the full-wave rectifier 4 is input, and the connection point of the resistors R1 and R2 is connected to the multiplier input terminal VDET. Also, the full-wave rectifier 4 is grounded via a resistor R3, and the connection point of the full-wave rectifier 4 and resistor R3 is connected to an inductor current signal generating input terminal IS of the power factor correction control circuit 10A. Furthermore, a series circuit of resistors R4 and R5 is connected in parallel with the load 8, and the direct current output voltage Vout the same as that of the load 8 is applied thereto. A feedback voltage input terminal FB of the power factor correction control circuit 10A being a terminal into which a detected value of the direct current output voltage Vout is input, herein, the connection point of the resistors R4 and R5 is connected to the feedback voltage input terminal FB, and a voltage signal wherein the direct current output voltage Vout is divided by a resistor is returned here.
Next, a simple description will be given of an operation of the heretofore described heretofore known power factor correction type switching power supply unit of FIG. 14.
The heretofore known power factor correction type switching power supply unit of FIG. 14 employs a control method called an average current control method, average current mode control, or the like, and the power factor correction control circuit 10A is one that sinusoidally controls a current flowing to the alternating current commercial power supply 2 side in the same phase as that of the alternating current input voltage, while stabilizing the direct current output voltage Vout. The feedback voltage input terminal FB of the power factor correction control circuit 10A is connected to an input terminal of a voltage error amplifier 14 together with a reference voltage source 12, which sets a voltage command value for the direct current output voltage Vout. The voltage error amplifier 14 generates a voltage error signal wherein the difference between the detected value (a divided voltage value in this case) of the direct current output voltage Vout and the voltage command value of the reference voltage source 12 is amplified. Then, the voltage error signal of the voltage error amplifier 14 is input into an Iy generator 16, and converted into a current signal Iy indicating a voltage error.
In the power factor correction control circuit 10A, its multiplier input terminal VDET being connected to a Vx generator 18 (a voltage to voltage converter circuit), the detected value (a divided voltage value in this case) of the output voltage of the full-wave rectifier 4 is input into the Vx generator 18, and converted into a voltage signal Vx. However, the Vx generator 18, being shown for the sake of a comparison with a power factor correction control circuit 10As in an embodiment to be described hereafter, is a simple wiring linking an input terminal and output terminal, and the voltage signal Vx is equivalent to the voltage of the multiplier input terminal VDET. Also, a constant voltage signal Vbias generated by an unshown circuit is input into an Iz generator 20, and converted into a current signal Iz.
The multiplier 22 multiplies the current signal Iy of the Iy generator 16 and the voltage signal Vx corresponding to the detected value of the output voltage of the full-wave rectifier 4, and makes this the value of a current command to a current error amplifier 24. An inductor current signal, wherein a voltage signal, which is an inductor current IL input via the inductor current signal generating input terminal IS and voltage converted in the current detecting resistor R3, is further inversion amplified in an inversion amplifier circuit 25, is input into the current error amplifier 24, along with an output signal Vmul of the multiplier 22, which is the current command value. A sawtooth wave or triangular wave carrier signal of a constant frequency that determines a switching cycle is generated in an oscillator circuit (OSC) 26, and input into a PWM comparator 28. In the PWM comparator 28 into which the carrier signal and the current error signal are input, the magnitudes of the signals are compared, a pulse width modulation (PWM) control signal is generated, and this is applied to the gate terminal of the switching element 6 via an AND circuit 32 and driver circuit 34.
Herein, an overcurrent protection (OCP) circuit 30 is connected to the inversion amplifier circuit 25, and limits the maximum value of the inductor current IL. Herein, when an inductor current exceeding a predetermined threshold value flows, an overcurrent limit signal L (Low) is input into the AND circuit 32, and the output of the AND circuit 32 compulsorily becomes L. As a switching signal is output to the output terminal DO of the power factor correction control circuit 10A from the AND circuit 32 via the driver circuit 34, the switching element 6 is turned off on the output of the AND circuit 32 becoming L. By controlling the on-off timing of the switching element 6 in this way, it is possible to control the value of a current flowing to the capacitor C1 via the diode D1. Actually, in the voltage error amplifier 14 and current error amplifier 24, a feedback constant setting circuit is connected between the input and output terminals, but a depiction of both feedback constant setting circuits is omitted from FIG. 14.
A power factor control circuit shown in FIGS. 7 and 8 of JP-A-2002-176768, and a self-exciting type power supply circuit using it, employ the heretofore described method called the average current control method, average current mode control, or the like. Also, a description is also given in JP-A-2002-176768 of a heretofore known overcurrent protection (OCP) and overvoltage protection (OVP).
However, in a power factor correction type switching power supply unit in which the switching frequency is fixed, the maximum value of the inductor current IL flowing through the inductor L1 is limited by the overcurrent protection (OCP) function. Herein, the overcurrent protection (OCP) function operates at a start-up time or a time of an excessive load, and a control is carried out in such a way that the inductor current does not exceed the tolerated maximum value. On a condition arising wherein the on duty (the on time ratio) of a pulse signal supplied to the switching element 6 in this condition exceeds 50%, it may happen that a sub-harmonic oscillation (a phenomenon wherein the on duty of the switching element 6 is unstable and wavers) occurs.
FIG. 15 is a timing diagram showing a signal waveform illustrating a sub-harmonic oscillation occurring when the OCP function operates at a switching element on duty of 50% or higher.
As shown in FIG. 15, when limiting the peak value of the inductor current IL at an OCP threshold value Is, although the angle of inclination of the current rise in an on period Ton, and the angle of inclination of the current fall in an off period Toff, do not change in switching periods T1 to T4, the ratio (on duty) of the on period Ton and off period Toff changes. This kind of phenomenon is called a sub-harmonic oscillation, and when the sub-harmonic oscillation occurs, it may happen that a load current becomes unstable. Also, when the sub-harmonic oscillation is occurring, it may happen that a ripple voltage included in the output voltage increases, or that a current change enters an audible region of 20 kHz or lower, and these are seen as a squeaking problem.
Although limiting a peak current is also effective in preventing a saturation of the inductor, there is a problem in that the heretofore described sub-harmonic oscillation cannot be avoided.
Also, as another protective function of the power factor correction type switching power supply unit, it is conceivable, by using an overvoltage protection circuit having the heretofore described overvoltage protection (OVP) function, to protect in such a way that the output voltage does not exceed the tolerable voltage of the load 8 for any reason.
FIG. 16 is a circuit diagram showing another example of the heretofore known power factor correction type switching power supply unit, this one including an overvoltage protection circuit, and FIGS. 17A and 17B are timing diagrams showing a signal waveform for overvoltage protection at a start-up time of the heretofore known power factor correction type switching power supply unit shown in FIG. 16. In the power factor correction control circuit 10B of FIG. 16, parts corresponding to those of the power factor correction control circuit 10A of FIG. 14 are shown with the same reference numerals and characters.
The heretofore known overvoltage protection circuit, on a load fluctuation or AC input voltage fluctuation occurring, and the output voltage or input voltage becoming excessive, causes the overvoltage protection (OVP) function using the overvoltage protection circuit to operate for the switching element, causing the switching element switching operation to stop completely (the switching element is turned off) after a time, or instantly. The power factor correction control circuit 10B of FIG. 16 is an example wherein the overvoltage protection circuit 40 carries out an overvoltage protection with respect to the output voltage. For example, after the start-up is started at a time t1, on a feedback voltage to the feedback voltage input terminal FB reaching a threshold voltage Vth of the overvoltage protection circuit 40 at a time t2, as shown in FIG. 17B, the switching element 6 stops operating. In this kind of case, the current flowing through the inductor L1 rapidly becomes zero, after which a resonance between the inductor L1 and a parasitic capacitor of the switching element 6 starts, and the kind of resonance current shown in FIG. 17A flows.
On the change of the resonance current at this time entering the audible region of 20 kHz or lower, it is seen as the same kind of squeaking problem as in the power factor correction control circuit 10A of FIG. 14 in the power factor correction control circuit 10B of FIG. 16 too.
FIGS. 18A and 18B are timing diagrams showing a signal waveform at a time of a cancellation of the overvoltage protection. As shown in the diagram, on the direct current output voltage Vout that exceeds a threshold value Vth1 at a timing t2 subsequently reaching a timing t3 at which it drops as far as a cancellation voltage Vth0 of the overvoltage protection (OVP), the stopped condition of the switching element 6 is cancelled, and the switching is restarted. In this case too, depending on the output voltage from the voltage error amplifier 14, it may happen that a large current flows through the inductor L1. Then, there is a problem in that a squeaking occurs due to a large current suddenly starting to flow through the inductor L1.
When carrying out a peak current limitation using the overcurrent protection (OCP), it is a cause of a sub-harmonic oscillation occurring, bringing about a squeaking. Also, as a squeaking due to a resonance current also occurs when stopping the switching element 6 using the overvoltage protection (OVP) function, a squeaking also occurs at a time of restarting after stopping the switching operation. That is, although the overvoltage protection (OVP) function stopping the switching element 6 is effective from the point of view of curbing a sudden rise of the output voltage, it is difficult to avoid the occurrence of an accompanying squeaking.